1. Field of the Invention
The present invention relates generally to an apparatus and method to obtain an image of a patterned substrate, and more particularly to an apparatus and method to raster scan a charged-particle beam over a patterned substrate on a continuously moving stage.
2. Description of Related Art
The traditional charged-particle beam imaging system, such as a Scanning Electron Microscope (SEM), generates images by raster scanning a primary charged-particle beam such as an electron beam (e-beam) over a sample held on a stationary stage. Referring to the drawings, FIG. 1 illustrates a charged-particle beam microscope 100 according to the prior art. A primary charged-particle beam is generated from a charged-particle beam source 110 which may be, such as an electron beam gun. The primary charged-particle beam is condensed by a condenser lens module 120 and focused by an objective lens module 130 to form a charged-particle beam probe 140. A deflection unit 150 scans the charged-particle beam probe 140 in lines across the surface of a sample 195 on a sample stage 190. It is noted that the one dimensional line scan call be converted to a two dimensional raster by offsetting the beam center, or by moving the stage 190 properly in an orientation perpendicular to the line-scan direction. After the bombardment of the charged-particle beam probe 140 on the sample 195, secondary charged particles 160, such as secondary electrons, are emitted from the sample 195 and, along with the backscattered charged particles, such as backscattered electrons, are collected by a charged-particle detector 170. Since the amount of secondary charged particles is modulated by surface topography or voltage of the scanned area, a two dimensional image representing the topography contrast or voltage contrast is obtained. The sample 195 may be a patterned substrate such as a wafer, a lithography mask or a semiconductor device and so on, or any combination thereof.
FIG. 2(a) illustrates a raster-scan operation in accordance with traditional prior-art principles. As shown, the raster scanning is performed by repeating line scans N times with each line advancing in a direction perpendicular to the line-scan direction. FIG. 2(b) illustrates the formation of an image of a raster-scanned substrate in accordance with the traditional art. Secondary and/or backscattered electrons are collected by a detector or detectors. Detector output signal is sampled at even timing intervals during the line scan, yielding a line matrix 201 of M pixels. Combining line pixel matrixes for all line scans forms a 2-dimensional pixel matrix 202, called a frame, wherein a frame represents the image of the raster-scanned area of the substrate being imaged. It is noted that the size of an image is referenced as a Field of View, or FOV, hereinafter.
In an actual raster scan, after reaching the last pixel of a line, the primary charged-particle beam traverses back to the starting point of the next line. The extra time required/spent for this fly-back is called line overhead. For simplicity of explanation, a line scan is represented only by the effective line scan in the following figures, but the line scan time or line scan repetition period actually (e.g., preferably) will be measured from the beginning of one line scan to the beginning of the next line scan within one frame, which by default includes the fly-back time or overhead time. Fly back time also exists in repeating frames. Frame time or raster-scan repetition period is measured from the beginning of one frame to the beginning of a next repeating frame, which by default includes the fly-back time or overhead time.
In order to improve the quality of the image, two types of image averaging methods, Line Averaging and Frame Averaging, are often employed.
Line Averaging is performed by repeating the line scan multiple times at the same position before advancing to the next line, thereby acquiring P matrixes of pixels for each image line. Averaging the line matrixes, pixel by pixel, yields an averaged line matrix. Combining all averaged line matrixes forms a line-averaged image of 2-dimensional pixel arrays.
Frame Averaging is performed by repeating the identical raster scan a designated number of times, S, with the stage held at a stationary position. This process generates S sets of 2-dimensional pixel matrixes. Averaging these matrixes, pixel by pixel, forms a single image of 2-dimensional pixel matrix, which is a frame-averaged image. Frame averaging can be applied to line-averaged frames.
A charged-particle beam inspection system based on scanning electron microscope (EB Inspector) typically acquires inspection images in either of two image acquisition modes, one known as “Step-and-Repeat” mode and the other known as “Continuous-Scan” mode.
For an inspection to be performed, a user specifies the certain areas on the pattern of the substrate (i.e., wafer or mask) to be scan-imaged. These areas are called Areas of Interest (AOI). The EB inspector acquires electron beam images covering an AOI and processes the images to identify abnormalities of the patterns or alien objects on the pattern.
In Step-and-Repeat mode, a series of images is acquired in a sequential manner. FIG. 3 illustrates Step-and-Repeat mode imaging covering an AOI on a substrate in accordance with the traditional art. Taking each image 301, the stage whereupon the substrate is secured for imaging is moved along a stage stepping direction so that the center of the imaging area of the pattern is brought to the center of electron optical axis (a small error or offset is usually tolerable and managed by the system). As a result, the imaging action of interested areas is stepped as required, for example, as illustrated by arrow 302. When the movement is settled, such that, for example, the stage is at/in a stationary position, the charged-particle beam is raster-scanned over the imaging area. A 2-dimensional array of pixel data representing the image of the scanned area thus can be obtained. The stage then steps forward to the next stationary position. This type of process is repeated until a desired AOI 303 is covered. The image averaging methods, Line Averaging and Frame averaging, are often employed to improve the image quality to achieve the required inspection sensitivity.
Throughput of the inspection available for the system operating in Step-and-Repeat mode is largely limited by the image FOV and stage stepping time. Image FOV determines the total number of stage steps required for covering a given AOI, while stage stepping time depends mainly on the stepping distance and tolerable position error. Stage stepping time is purely an overhead time and generally falls into the range between 0.1 to 0.5 seconds. It is important to reduce the number of steps and stage stepping time.
A relatively recent EB Inspector, which operates in Step-and-Repeat as the default image acquisition mode, addressed this throughput issue by introducing an electron optics design to achieve Large Field of View (LFOV). FIG. 4(a) illustrates a Step-and-Repeat mode using LFOV to improve the throughput in accordance with the traditional art. If the LFOV is L times larger than a normal FOV, the number of stage steps required to cover the given AOI will be reduced by a factor of L2. As shown, with other settings kept the same as in FIG. 3, image 401 is acquired using an FOV three times larger in size than that used for image 301. That is, if image 301 is of a size of single FOV, then image 401 is of a size of 3 FOV. The imaging action again steps as required, as illustrated by arrow 402. As a result, it can be seen in FIG. 4(a) that, by using LFOV image 401, only three stage steps are required to cover the same AOI 303. As compared to the greater number of stage steps needed in FIG. 3 using LOV image 301, the throughput of the embodiment of FIG. 4(a) would appear to offer improvement.
In practice, a LFOV is divided into multiple sub-FOV fields for beneficial low noise and high speed raster scanning. While each sub-field is imaged with traditional raster scanning, a relatively low frequency step signal, which is synchronized with sub-field frame rate, is superimposed onto the raster-scan signal for positioning or stitching each sub-field sequentially. FIG. 4(b) illustrates a Step-and-Repeat mode raster-scan imaging operation using a LFOV with multiple sub-fields in accordance with the traditional art. As shown, LFOV image 403 includes four sub-FOV fields 404 captured with one stage move. The imaging action steps again as illustrated by arrow 402. It is noted that usually the fly-back time of the charged-particle beam, such as an electron beam, between each sub-field raster scan is negligible. It is also noted that the number of sub-fields does not change the number of stage steps. The number of stage moves depends mainly on the size of LFOV. For the implementation of a LFOV 12 times larger than a normal FOV, the number of stage steps in optimal cases can be reduced by a factor of 144. However, as endless demand for higher throughput in EB inspector applications pushes toward higher pixel rates, with image raster time getting shorter and shorter, stage stepping time still remains as the top throughput-limiting factor in Step-and-Repeat mode imaging.
It may be noted that in FIG. 4(b) the width of the sub-FOV 404 is much smaller than its height. Line Scan is required to be driven by high speed (i.e., high bandwidth) electronic/electric circuitry, where enlarging the dynamic to extend the line length is limited by the requirement to maintain noise level to a required specification. Also, the beam scan optical scheme needs to be constructed in a simpler fashion, where the scan range is limited to keep the beam under tolerable blur. Moving the line scan to the next line can require much slower electronics, which may allow a designer to construct much larger dynamic ranges, staying in the noise tolerance. The slower operation may also allow a designer to choose a more sophisticated beam deflection scheme(s), which can allow the nominal beam path to be minimally impacted relative to the beam property when the line scan is moved gradually from the top to the bottom of the sub-FOV by a large distance.
FIG. 5 illustrates Continuous-Scan mode imaging in accordance with the traditional art. Unlike Step-and-Repeat mode, which relies on raster scanning to achieve both line scan and line-to-line stepping to cover a full frame of image, as shown, in Continuous-Scan mode the stage moves at a constant speed. More particularly, the stage moves at a constant speed along a stage-moving direction 502 while an e-beam repeatedly line scans at a fixed offset from optical axis in a line-scan direction 501 usually perpendicular to the stage-moving direction 502. The stage continuously moves for the imaging action to be continuously performed, as illustrated by arrow 503, until a desired quantity (e.g., length) of image is acquired. This can form a relatively long image/frame. It is noted that in Continuous-Scan mode the sample is scanned at an equal pitch of the stage speed multiplied by the line scan period.
FIG. 6(a) illustrates an AOI being imaged in Continuous-Scan mode in accordance with the traditional art. As shown, a large AOI 601 can be covered by multiple long images formed by raster scanning in Continuous-Scan mode. It may be noted that the stage-moving direction alternates though the neighboring images, as shown by the curved arrows 602, known as a serpentine stage scan, to minimize stage-moving time between each image scan. The time period of such alternating stage movement is known as the stage turnaround time.
Continuous-Scan mode provides much higher throughput compared with Step-and-Repeat mode for a large AOI, because stage stepping times required in Step-and-Repeat mode can be significantly reduced. The number of stage turnarounds is only a function of the AOI height divided by the line scan width, thus the stage-scan direction is generally chosen to be parallel to the long side of the AOI rectangles.
The line scan width, that is, the height of the inspection image in Continuous-Scan mode, is limited by two factors: (1) image FOV of electron optic design; (2) high speed scan requirement and tolerable scan/detection noise. For inspection of a small AOI which is relatively narrow in width, for example along the stage-moving direction, the inspector has to stack up a number of inspection images to cover the height of the AOI, accumulating stage turnaround actions while the actual imaging time per image is small due to the limited width of the small AOI. Stage turnaround time is usually larger than the stage stepping time by approximately a factor of 0.7 to 2.0.
FIG. 6(b) illustrates imaging of a small AOI in Continuous-Scan mode and in Step-and-Repeat mode using LFOV in accordance with the traditional art. As shown, assuming AOI 603 has a height of 24K, if the height of the Continuous-Scan mode image is 2K pixels, 8 stage turnaround actions are required to cover AOI 603. On the other hand, with the use of LFOV and a size of 12K pixels, only 3 stage steps are required for the Step-and-Repeat mode operation. Therefore, the Step-and-Repeat mode with LFOV benefits the imaging of small AOI, as the number of stage steps can be less than the number of stage turnarounds in Continuous-Scan mode.
For scattered smaller AOIs or arrays of small AOIs within a die, Continuous-Scan mode wastes more time either on the non-AOI region with the stage moving at the constant speed of imaging, or with the stage frequently skipping the non-AOI region at a higher speed but then taking extra time to settle back to the constant speed of imaging before entering an AOI. In such cases, the original throughput advantage of Continuous-Scan mode over Step-and-Repeat mode often diminishes steeply and may even become worse than Step-and-Repeat mode.
FIG. 7(a) and FIG. 7(b) respectively illustrate the imaging of scattered small AOIs 701 in Continuous-Scan mode and in Step-and-Repeat mode using LFOV in accordance with the traditional art. As shown, again with the use of LFOV, Step-and-Repeat mode benefits the imaging of small AOIs 701, as the number of stage steps (3×3=9) can be less than the number of stage turnarounds in Continuous-Scan mode (6+6+3=15).
Accordingly, what is needed is a system and method that overcomes the above identified issues. The present invention addresses such a need.